When you put the new capacitors in, an easy way to tell if you oriented them correctly is to see if the vent hole is visible. Computer grade electrolytics have a small rubber cover over a hole about 3/16" in dia on the bottom. This vent allows vapors to escape to prevent the case from rupturing in the event of a catastrophic failure. Mcintosh punches a third hole in the chassis for each computer grade electrolytic to keep this vent exposed. From the inside of the chassis you should see the little rubber cover if oriented properly.
Also, when you remove the old capacitors, remember the stacking order of any solder posts and insulating washers (if any) under the screw head for each of the two terminals. This is usually a no-brainer since the wires are stiff and hold their relative positions pretty closely.
To summarize other posts:
. a rated voltage higher than 40v is OK but not lower
. a higher capacitance is preferred but stay =< 1.5x of the original (higher that 1.5x might cause an input current rush problem and blow fuses. Remember that big capacitors look like a black hole for current just after the power switch is flipped)
. a lower height is Ok and a larger height is OK too if you are not worried about appearances (the perferated cover may not fit)
. diameter is critical and stud spacing (center to center) is critical
. The studs on Mcintosh capacitors (and the most common I've seen) have a black plastic spacer that raises the top edge of the screw hole to about 1/4" above the bottom surface. Don't get a capacitor that has the threaded hole almost flush with the bottom and no spacer (somewhat rare). The black plastic spacers keep the two studs of the capacitor insulated from the steel chassis
. If you are really picky and it matters to you, remember that Mallory caps are blue not silver like the originals. Look at various ebay auctions for amps and you will undoubtly see some blue caps.
I draw this experience from working on 2200's and 2205's but I am sure 95% of it will also apply to 250's, 2505's, 2100's and 2105's (not the twist prong caps).
John